The present invention relates to a photoconductive target of an image pickup tube and, more particulaly to a target having a high photo-sensitivity even in an infrared range and a manufacturing method thereof.
Image pickup tubes which utilize cadmium selenide (CdSe) as a material for a photoconductive layer of a target, e.g., "Chalnicon" (trade mark; available from TOSHIBA), are commercially available. The target of such an image pickup tube has a multilayer structure consisting of a CdSe layer and a high resistance layer of arsenic sulfide (As.sub.2 S.sub.3) or arsenic selenide (As.sub.2 Se.sub.3) deposited thereon. Since the target has a quantum efficiency of about 1 in the visible light range and therefore has a high sensitivity, "Chalnicon" is very suitable for a monochrome or a color image pickup tube. However, in "Chalnicon", the limit of spectral sensitivity of long wavelengths is close to 700 nm, and photo-sensitivity in the infrared region exceeding this limit is insufficient. When an image pickup operation is performed at low illuminance, e.g., for monitoring roads at night, the interior of tunnels or warehouses, an image pickup tube having a high sensitivity, not only in the visible light region but also in the infrared region, must be used. Therefore, a demand for such an image pickup tube has arisen.
An example of an image pickup tube target having a high sensitivity in the infrared region is disclosed in Japanese Patent Disclosure No. 57-208041. This target has an evaporated layer which consists of a mixture of CdSe and CdTe and has CdSe as its major component. However, this target does not have a satisfactory dark current characteristic and a sufficient photo-sensitivity in a long wavelength region.